When the red lights flash.
The stopping rule, its single exception, and why that exception is so often misread.
Amber first, then red.
Overhead amber lights mean the bus is preparing to stop. That is your cue to slow down and get ready, not to hurry past before it stops. Flashing red overhead lights, or an extended stop arm, mean stop. If you cannot see the overhead lights clearly but the stop arm is out, the stop arm is enough on its own.
Behind the bus, you always stop.
Traffic approaching a stopped school bus from behind must stop at least 20 metres back, and no road layout changes that. A median does not release you, a multi-lane road does not release you, and moving into the lane furthest from the bus does not release you. The distance exists so children crossing in front of the bus are not approached from behind.
The exception is narrower than people think.
The median exception applies only to traffic travelling in the opposite direction, and only where a raised median or physical barrier separates the roadways. Painted lines are not a median, however wide or solid they look. On an undivided road, both directions stop. This is the single most misread rule in the whole knowledge test.
It does not depend on a school being nearby.
The requirement applies on a rural highway as much as on a city street, in a signed school zone or far from one, during school hours or outside them. It also does not depend on your seeing a child. The rule attaches to the bus and its lights, not to the surroundings or to what you happen to notice.